This invention relates to a scheme for sensing small amounts of charge and more particularly relates to a balanced differential charge sensor with sense nodes capacitively decoupled from the output nodes of charge storage elements and means for biasing and resetting the output nodes.
The advent of charge-transfer systems as means of accomplishing data storage and retrieval has established the need to detect accurately relatively small amounts of charge. Bucket-brigade devices, charge-coupled deviced (CCD's) and similar devices are capable of storing and transferring charge with magnitudes smaller than 50 .times. 10.sup..sup.-15 coulombs. Traditional techniques such as floating gate amplifiers or gated charge integrators are not totally suitable for measuring such small charges or for discriminating between small charges, particularly if the sense node capacitance is large. The use of a gate to decouple the incoming charge from a high-capacitance distribution line from a sense node is disclosed in Engler et al, "A Surface-Charge Random-Access Memory System", J.SSC, SC-7, No. 5, pp. 330 et seq., and in Heller et al, "High-Sensitivity Charge-Transfer Sense Amplifier", Digest, ISSCC, Feb. 13, 1975, pp. 112 et seq. At high data rates this technique has been found to be subject to "pattern sensitivity", i.e., the detected charge will vary as a function of the history of the information signal. Such pattern sensitivity is intolerable in a device which must be capable of operating with any conceivable information sequence which may be stored therein and retrieved therefrom.
An underlying problem which prior sensing schemes encountered in transferring small amounts of charge when output node capacitances were large was that a small amount of charge would not produce sufficient transconductance during transfer to allow the charge to be completely transferred in a reasonable amount of time. A technique to circumvent the slow response of a large capacitance to a small charge transfer is to precharge or bias the output node. If the precharge is suitably achieved, then even a small signal charge will be transferred quickly. See U.S. Pat. No. 3,764,906 issued to Heller and entitled "Stored Charge Detection by Charge Transfer." The precharge must be carefully generated, thereby necessitating considerably support circuitry.
Another problem in measuring small amounts of charge is that the measurement means must be able to discriminate between small charges whose actual magnitude may vary as a result, for example, of processing variations or changes in operating conditons such as temperature, etc. Also, the sensitivity of the measurement means must be high enough and must track process and operating conditions so that accurate measurement can be made over the range of expected charge magnitudes. This requirement inherently increases the number of components and complexity of the measurement means. A technique that is used to moderate this problem is to utilize a balanced measurement means. This has the effect of reducing changes in the absolute value of device parameters to only the effective differences between compared quantities with these differences being small for matched devices. See, for example, copending application Ser. No. 492,649 for "Dynamic Reference Voltage Generator" assigned to the same assignee as this application, and U.S. Pat. No. 3,760,391 issued to Y. L. Yao for "Stored Charge Memory Detection Circuit." Such schemes generally require particularized and therefore complex circuit architecture that can limit the range of applications.